The present invention relates to treating material and, more specifically, the method and apparatus for classifying and imparting controlled energy into the forming pellets.
Problems in operation of mineral treating apparatus, such as a Portland cement producing system, relate back to the pellet input thereto. Such a system includes a traveling grate on which the material to be treated is deposited and carried through a drying or conditioning chamber wherein the material in the form of pellets is dried. From the drying chamber the pellets are moved to a preheat chamber and thence progressed into a kiln for final burning and thence to a cooler.
Raw material is fed to the system from a pelletizer pan which operates as a positive displacement machine converting the raw material into green pellets or wet balls that are discharged as feed input to the system. The pelletizer pan retains the undersize pellets or balls until they grow to reach the final mature size before discharging. The green pellet or ball forming process in the pan consists basically of: (1) addition of water to the feed material; (2) formation of the nuclei or seed pellets; (3) growth and densification of the seed pellets; and (4) final growth and densification. In each revolution of the pan, the agglomerates and fines are lifted to the maximum height and then fall freely to the lower section of the rotating pan. In the load, large pellets circulate on top of the smaller pellets over a relatively small area in the lower outside portion of the pan close to the rim at the discharge point. Smaller pellets lifted under the larger pellets circulate over a wider area in the pan and become exposed to the water spray and incoming fine material being fed into the pan. Thus, the smaller pellets grow and are then again lifted under the larger pellets.
In the pelletizer pan presently in use, pellet size is a function of: (1) the location of the water spray and feed entry point which controls the rate of pellet seed formation and the degree of pellet growth; (2) uniformity of moisture addition, i.e., a more uniform addition of moisture reduces the variation in the size of the pellets; (3) moisture level--pellet size increases at higher moisture levels; (4) peripheral speed--with high peripheral pan speeds, pellet size increases; and (5) retention time of the pellets in the pan--pellets produced in a relatively short retention time normally have a higher moisture content.
Generally the aforementioned variables all effect the material flow pattern and, hence, pellet seed formation as the degree of moisturizing the fines varies. Moving the larger pellets into the area of the new feed material and water causes the pellets to go out of phase and increase in size and thereafter rapidly decrease in size and thence cycle again. As a result, pan loading decreases momentarily, thus feed output decreases. This is true because as the size of the pellet increases due to increase in the moisture level, the pellets move or flow farther outward occupying more pan volume. Also, increasing the pan peripheral speed causes the pan load to shift outward into the new feed area which also causes an increase in the size of the pellets.
Since the material in the pelletizer pan is sensitive to any kind of change in the aforementioned variables, the pellet feed output of the pan is likewise sensitive to seed pellet change. As a result, pellet pan output is not stable and, thus, the operation of the entire material treating system cycles in accordance with cycle output of the pelletizer pan which has been found to be of substantial 20-minute duration, or the pelletizer pan output is stable but pellet quality is not at a desired level.
It is, therefore, an object of the present invention to provide an improved method and apparatus for classifying pellets formed in a pelletizer pan.
Still another object of the present invention is to provide an improved method and apparatus for imparting controlled energy to the pellets formed in a pelletizing pan to produce a pellet of improved quality and increase pan capacity.
Yet another object of the present invention is to provide an improved method of stabilizing the pellet output of a pelletizer pan.
A still further object of the present invention is to provide an improved method of controlling the classifying of the forming pellets which permits easier direction of water to the dry fines to control the moisture content of the formed pellets.
Another object of the present invention is to provide an improved pelletizer pan which may be driven at a constant speed.